Oven apparatus

ABSTRACT

A device, for making a furnace atmosphere for the chemicometallurgical influencing of workpieces during heat treatment, consisting of a reaction chamber surrounded by a heat protection jacket in which fuels, at a temperature between 750* and 1100*C, react to form a gaseous mixture, the reaction chamber being arranged outside a heat-treatment furnace space and the gaseous mixture being admitted to the furnace space, is characterised in that the gaseous mixture is heated inside an opening in the wall bounding the inner chamber where heat treatment takes place, by means of a heating device, to a temperature between 750* and 1100*C.

ohring 1 Nov. 4, 1975 OVEN APPARATUS [76] Inventor: Werner iiohring, 40 Materbomer Allee, D-419 Kleve, Germany [22] Filed: Sept. 13, 1973 [21] Appl. No.: 397,164

[30] Foreign Application Priority Data FOREIGN PATENTS OR APPLICATIONS 292,412 3/1966 Australia 23/281 623,164 5/1949 United Kingdom 266/5 R X Primary ExaminerRoy Lake Assistant Examiner-Paul A. Bell Attorney, Agent, or FirmWilliam Anthony Drucker [57] ABSTRACT A device, formaking a furnace atmosphere for the chemico-metallurgical influencing of workpieces during heat treatment, consisting of a reaction chamber surrounded by a heat protection jacket in which fuels, at a temperature between 750 and 1100C, react to form a gaseous mixture, the reaction chamber being arranged outside a heat-treatment furnace space and the gaseous mixture being admitted to the fumace space, is characterised in that the gaseous mixture is heated inside an opening in the wall bounding the inner chamber where heat treatment takes place, by means of a heating device, to a temperature between 750 and 1100C.

1 (Claim, 2 Drawing Figures U.S. Patent Nov. 4, 1975 Sheet 1 of 2 3,917,238

U.S. Patent N0v.4, 1975 Sheet20f2 3,917,238

OVEN APPARATUS The invention relates to a device, for establishing a furnace atmosphere for the chemico-metallurgical influencing of workpieces during heat treatment, consisting of a reaction chamber surrounded by a heat protecting jacket, in which fuels react at a temperature between 750 and llC, the reaction chamber being arranged outside a heat treatment furnace space and the gaseous mixture is admitted to the furnace space.

One purpose of the invention is to prepare a gaseous mixture suitable as furnace atmosphere, with low outlay on construction, power and maintenance. A further aim of the invention is to reduce the risk of explosion of furnace installations in which inflammable furnace atmospheres are used. The invention furthermore sets out to improve the transfer of carbon from the furnace atmosphere to the workpieces and the ability to judge the effect of the furnace atmosphere on the workpieces undergoing heat treatment.

It is well known that a heated catalyst bed can be arranged outside the wall bounding the inner chamber of the heat treatment furnace and that the gaseous mixture can be passed uncooled into the inner chamber of the furnace (journal Blech 1968 No.11, page 584). In this arrangement, however, there is the drawback that after leaving the heated catalyst bed, on its way to the interior of the furnace, the gaseous mixture traverses unheated cooler zones and thus cools down. In the case of most of the gaseous mixtures containing carbon monoxide that are used, there occurs with slow cooling a troublesome liberation of soot, associated with a change in the composition of the gas. This arrangement of the catalyst bed together with the introduction of the gaseous mixture into the interior of the furnace at an indefinite temperature which it is not possible to influence has not therefore been able to gain much foothold in practice.

In todays usual forms of reaction chambers for the purpose of establishing furnace atmosphere, the emerging gaseous mixture is quickly cooled with a refrigerant and admitted in the cold state into the interior of the heat treatment furnace. There is no liberation of soot. The cooling and reheating of the gaseous mixture is uneconomic as regards both the constructional outlay required and the power consumption.

lt was realised that, with a gaseous mixture admitted cold into the furnace space, fewer heat-consuming carbon dioxideforming intermediate reactions take place in the furnace space and that the establishment of the chemical equilibrium takes place in a second stage of the reaction, which is slower and as a rule no longer takes place completely. The figures for carbon dioxide that are'used'for assessing the effect of carburisation are therefore too high, the starting point is a carbon content corresponding to the chemical equilibrium.

In the case of the transfer of carbon to steel, which has recently been applied, in the temperature range between 550 and 600C, the carbon transfer between the furnace atmosphere and the workpieces takes place very inertly owing to the slow speed of the reaction caused by the low temperature. In this temperature range there occurs to a great extent uncontrollable gas reactions on the surface of the workpieces, e.g. oxidation, through traces of oxygen, which also affect the carbon transfer.

2 The gaseous mixtures prepared in a reaction chamber for the purpose of use as furnace atmosphere are as a rule inflammable. On passing such a mixture into the chamber of a heat treatment furnace filled with air, an 5 explosive gas-air mixture is formed unless a stable flame is at once formed at the point of introduction. This flame must burn for a sufficient time until the proportion of oxygen in the inner chamber of the furnace has decreased to such a degree that after the flame has been extinguished through oxygen deficiency an explo sive gas mixture can no longer form. In the case of the known devices this is only ensured if the inner chamber has a temperature above the ignition point of the gaseous mixture.

The invention is based on the task of providing a simple and economically operated device making it possible to arrive at the necessary state of reaction of the gaseous mixture in the furnace chamber without troublesome intermediate reactions, and to obtain reliable flame formation at the point where the gaseous mixture is admitted to a furnace space filed with air.

To solve this problem, according to the invention the gaseous mixture is heated, within an opening in the wall bounding the inner chamber of the heat treatment, with a heating device to a temperature between 750 and 1 100C.

The gaseous mixture passing according to the invention into the furnace chamber has a high content of perceptible heat as a result of being heated as far as the point of entry. It has been recognised that through this energy content the following advantages are obtained:

On the establishment of chemical equilibrium of the entering gas mixture the troublesome intermediate reactions are dispensed with, since it brings with it sufficient heat to obtain the necessary state of reaction in a single reaction stage. This applies both to the establishment of the watergas equilibrium and to the reaction of hydrocarbons to form carbon monoxide. lt is of special importance here that the gaseous mixture entering the inner chamber of the furnace should come into reaction contact at the highest possible temperature with the gaseous mixture already available in the inner chamber.

Owing to the intense molecular movement, the hot gaseous mixture transfers more carbon to the surface of the workpiece through the cracking of carbon monoxide. Moreover, free oxygen penetrating into the furnace chamber reacts for preference with the hot gaseous mixture before a layer of oxide is formed on the surface of the workpiece, which also impedes the transfer of carbon. These advantages are felt particularly in the low-temperature zone of heat treatments.

The temperature of the gaseous mixture admitted to the inner chamber of the heat treatment furnace is regulated and monitored at values between 750 and 1 100C. These temperatures are above the ignition temperatures of the gaseous mixtures used as furnace atmosphere'Theassurance is therefore furnished that the hot gaseous mixture entering the furnace chamber, on coming into contact with air, at once forms a flame before an explosive mixture can be produced. The sensible-heat" of the gaseous mixture imparts to the flame particularly stable combustion properties so that the flame burns for a sufficient length of time until the oxygen content in the furnace chamber is so low that there is no'longer any risk of explosion. I

Even if air enters a furnace chamber filled with inflammable g as, immediately on meeting it, the oxygen 3 reacts with the gaseous mixture that is above the ignition temperature, so that again an explosive mixture cannot form. With the device according to the invention it is therefore possible to operate furnace chambers with an inflamable gaseous mixture above the ignition temperature of same without risk of explosion.

With the usual arrangement of the inlet aperture transversely in the outer wall, the heated spot of introduction occupies the smallest possible area of the furnace wall aand practically does not impair the temperature control of the material being heat treated in the furnace, by heat radiation.

In one practical form of the invention the gaseous mixture is passed in a heated pipe from the reaction chamber to the opening in the furnace wall. Through this measure, the usual cooling of the gaseous mixture to the reaction chamber is dispensed with.

In another practical form of the invention, the reaction chamber ends direct in the aperture of the furnace wall. Through this measure a special supply pipe between the reaction chamber and the furnace wall and its heating device, is dispensed with.

If the gaseous mixture is to be admitted to an inner space of a heat treatment furnace which is provided with heat protection, a particularly suitable and preferential form of carrying out the invention is possible. A catalyst bed acting as reaction chamber is arranged inside the opening of the heatinsulated wall.

Through this measure, the heat insulating wall of the heat treatment furnace is used at the same time as a heat-protecting jacket for a heated catalyst bed. Furthermore, no special chamber and no special heating device is needed for keeping the gaseous mixture warm. With this arrangement of the catalyst bed, the gas outlet points lie within the plane of the geometric shape of the inner chamber of the heat treatment furnace. The gaseous mixture thus passes directly into the inner space of the furnace at the temperature of the heated catalyst bed.

The thickness of the heat-insulating outer wall bounding the inner wall of a heat treatment furnace is between 25 and 35cm. The usual length of heated catalyst beds is at least three times this thickness of outer wall. In order not to impede access to the furnace installation by protruding portions of the plant, or to increase the space required, it is proposed according to the invention to heat the catalyst bed solely in the .re gion of the heat insulating outer wall.

It has been found with some surprise that the shortening of the catalyst bed involved thereby is possible without disadvantages, although as a rule elongated forms of construction are conventional.

In order to transfer the necessary heatto the shortened length of bed it is proposed according to the invention to effect the heating of the catalyst bed by a heating conductor consisting of an electric heating resistance material, traversed by electric current, and in contact with the pieces of catalyst and the reacting gaseous mixture.

cross-section of the heating conductor affords further.

advantages. Carbon passing from the gaseous mixture into the outer surface of the heating conductor quickly travels to the inner surface. There the carbon reacts with the air present and there are no concentrationsof carbon that are detrimental to the material of the heating conductor.

The examples that are described below serve for further explanation in conjunction with the drawing.

FIG. 1 shows a diagram ofa heat treatment furnace with a practical form of heating according to the invention of the gaseous mixture on passing through the furnace wall.

FIG. 2 shows the particularly advantageous preferential form of execution with the arrangement and development of an electrically heated catalyst bed inside the furnace wall.

In the inner chamber 1 of a heat treatment furnace are the workpieces that are warmed under a furnace atmosphere, or after that cooled. This inner space therefore contains as necessary heating or cooling means as well as devices for circulating the gases, such,

as fans and gas guide walls. In this inner space 1 there ends, according to the form of execution shown to FIGS. 1, a pipe 2 which communicates with a reaction chamber 3. This pipe 2 is heated by a heating device 4 to a temperature between 750 and l,l00C. It is=surrounded by a heat protective jacket 5. Through this tube 2, which is also heated in the zone of the furnace wall. 6 flows'the gaseous mixture acting as furnace atmosphere from the reaction chamber 3 into the inner 7 space 1 of the heat treatment furnace.

i In the case of the device that is represented in FIG. 2,

the heat treatment furnace consists of an inner ch amber 1, which is bounded by a continuous heat-insulating wall 6, which at the same time also represents the outer wall of the furnace. In an opening 7, transversely in the furnace wall 6, there is arranged a catalyst bed 3 which is heated by electricity. A heating register 8 is accommodated inside the bed and is in direct contactwith the pieces of catalyst 9. Ithas a tubular cross-section. Two

tubular connections 10 leading outwards serve to apply the electric voltage. Through a pipe 11 a fuel or a fuelair mixture is admitted to the heated catalyst bed..The

gaseous mixture occurring during the reaction, passes through the outlet apertures 12 into the inner space 1. The catalyst bed 3 is, according to the invention,

heated in the zone of the thickness S of the heat insulating wall 6. The measurement S arises from the distance of the geometric shape of the inner space 1 from that of the surface of the outer wall and is not influenced by protruding or recessed parts ofthe furnace.

In the case of the example represented in FIG. 2, heating takes place exclusively in the region of the wall 6 (measurement S). The insulating parts (13) protruding from the wall 6 merely serve as protection against heat losses of the heated catalyst bed.

As raw material for making a gaseous mixture acting as the furnace atmosphere. hydrocarbon-air mixtures are generally fed to the reaction chamber. There are then produced gaseous mixtures containing carbon monoxide which are able to influence the proportion of carbon in steel workpieces. Through the use of ammonia, or an ammonia-air mixture, it is also possible to make mixtures of gas free from combined oxygen and from carbon.

in the event of high metallurgical demands on quality, steel workpieces are warmed in a gaseous mixture free from carbon and then exposed to a gaseous mixture containing carbon monoxide for the transfer of the carbon. In this way inequalities caused by the heating operation, and affecting the material, are avoided. The invention renders possible for this warming operation the explosion-proof use of ammonia as raw material for a furnace atmosphere free from carbon, it also makes it possible, by simple means, to adapt the properties of the furnace atmosphere to the different requirements of the materials during the heat treatment operation.

The teaching of the invention is based on recognition for the first time of the advantages of passing a gaseous mixture in the hot state from a reaction chamber into a heat treatment furnace chamber. The arrangement according to the invention of a catalyst bed, its size, form and nature of heating are not obvious having regard to the state of art.

sphere. Particularly progressive is the explosion-proof operation of furnace installations with the device according to the invention.

I claim:

1. Apparatus for the heat treatment of metal workpieces in a controlled atmosphere, comprising:

i. a furnace chamber to receive workpieces;

ii. a heat-insulating wall surrounding said furnace chamber, said wall having an opening therein extending all the way through said wall from the inside of the chamber to the outside of the wall;

iii. a catalyst bed disposed in said opening of said wall and extending from the inside line of the furnace wall to the outside line of the furnace wall and substantially filling the opening;

iv. electric heating elements disposed within said catalyst bed, said elements being pipes of which the outer surface is in contact with catalyst and of which the inner surface is open to the external atmosphere; and

v. means for introducing a fuel into said catalyst bed for passage therethrough into said furnace chamber. 

1. Apparatus for the heat treatment of metal workpieces in a controlled atmosphere, comprising: i. a furnace chamber to receive workpieces; ii. a heat-insulating wall surrounding said furnace chamber, said wall having an opening therein extending all the way through said wall from the inside of the chamber to the outside of the wall; iii. a catalyst bed disposed in said opening of said wall and extending from the inside line of the furnace wall to the outside line of the furnace wall and substantially filling the opening; iv. electric heating elements disposed within said catalyst bed, said elements being pipes of which the outer surface is in contact with catalyst and of which the inner surface is open to the external atmosphere; and v. means for introducing a fuel into said catalyst bed for passage therethrough into said furnace chamber. 